The long-term objective of this research is to understand the mechanisms that regulate the repair of chromosomal double-strand breaks (DSB), arisen during physiological processes or after ionizing radiation or chemotherapy. We aim to learn the impact of deregulated DSB repair on cellular mutagenic processes and genomic instability that promote carcinogenesis. Patients who carry germline mutations in the tumor suppressor gene BRCA1 have significantly increased risk of developing breast and ovarian cancer. Although numerous cellular processes have been ascribed to the function of BRCA1, why loss of BRCA1 leads to increased genomic instability and cancer predisposition is still unclear. Evidence indicates BRCA1 may play a central role in cellular response to DSBs. Through functional interaction with other DNA damage signaling and repair proteins including Chk2, Mre11 and Ku80, BRCA1 may orchestrate the repair of DSBs by competing mechanisms of homologous recombination (HR) and nonhomologous end-joining (NHEJ). This study will focus on determining the role of BRCA1 in controlling NHEJ, the predominant DSB repair process in mammalian cells. The main hypothesis is that BRCA1 maintains genome integrity by promoting the Ku80-dependent NHEJ that repairs DSBs more precisely and by preventing the Mre11-dependent NHEJ that typically repairs DSBs imprecisely with high mutagenic potential. With an array of chromosomal repair substrates as the functional readout for different nonhomologous repair sub-mechanisms, we will use genetic and siRNA approaches to (1) study the function of BRCA1 in regulating the two different NHEJ subpathways at the chromosomal level within genetically well- defined human and murine cell lines, (2) determine the role of BRCA1 phosphorylation by Chk2, a DNA damage response kinase, in controlling NHEJ processes, and (3) identify whether protein-protein and protein-DNA interaction are the molecular mechanisms by which BRCA1 prevents the mutagenic NHEJ through Mre11 and promotes more precise NHEJ through Ku80. An understanding of the defects in DNA repair that promote carcinogenesis and affect tumor response to radiotherapy and chemotherapy may offer novel avenues for both cancer prevention in individuals carrying BRCA1 mutations and tailored therapy in patients with BRCA1-deficient tumors.